Warning signs have been installed around Lake Bullen Merri as an algal bloom is affecting the water.
The Department of Environment, Land, Water and Planning (DELWP) has taken a sample from the lake to confirm whether the type of algae present poses a public safety risk.
As a precaution, the public should avoid all contact with the water, including activities such as swimming and boating.
Assistant chief fire officer Andrew Morrow said blooms could occur in hot and dry conditions or when nutrient levels in waterways are high.
“We urge people to avoid any direct contact with affected water until the algae disperses and warning signs are removed,” he said.
“Anyone who comes into contact with contaminated water should immediately wash in fresh water.”
Pet owners are also asked to keep their animals away from the water.
“We will notify the public and remove signage once the lake is safe for use again,” Mr Morrow said.